Sheet registration systems deliver sheets of all kinds to a specified position and angle for a subsequent function within a printer, copier and other devices. The subsequent functions could include transferring an image to a sheet, stacking the sheet, slitting the sheet, etc. Conventional registration systems correct for skew and lateral offset. “Skew” is the angle of the leading edge of a sheet being transferred with respect to the direction of transfer. Lateral offset is the cross-process misalignment of the sheet being transferred with respect to the transfer path.
Skew contributors include the angle at which a sheet is supplied into the sheet drive apparatus, skew induced when the sheet is acquired by the feeder, and drive roller velocity differences between drive rollers on opposite ends of a common drive shaft. Lateral offset may be due to sheet supply location and sheet drive direction error. Sheet drive direction error is caused by the sheet drive shafts not being perpendicular to the intended sheet drive direction. This is a result of tolerances and excess clearance between drive shafts and frames, sheet transport mounting features and machine frames and machine module to module mounting.
In present day high speed copiers and printers, active registration systems are used to register the sheets accurately. In an active registration system, a sheet is passed over sensor arrays from which the sheet skew and lateral or cross process offset is calculated. In some registration systems, the sheet is then steered into the proper position by rotating drive rollers on opposite ends of a common drive axis at different velocities. This function must be performed within a specific time and distance, i.e. before the sheet passes out of the nip rollers. As the sheet is moved more rapidly to increase overall productivity, the time to register the sheet to correct for skew and lateral offset decreases. As the allotted time decreases, the speed and acceleration of the nip rollers increases. The increased speed and acceleration may result in a need for a larger motor to provide additional power. The increased speed and acceleration of the nip rollers may further result in early failure of the registration system.
Other known devices use a loop registration process. In accordance with a loop registration process, the leading edge of a sheet is brought into abutment against a non-moving nip and idler roller pair causing the sheet to bend. The leading edge of the sheet is thus aligned with the nip and idler roller pair by the elasticity of the sheet to correct skew. Thereafter, the nip and idler roller pair is rotated at a predetermined timing by a process or forward motion motor to move the sheet through the machine.
In such devices, a loop space for forming a loop is required which results in an increase in the size of the apparatus. In addition, when the skew of a sheet is too large for the space provided, a paper jam may occur due to the buckling of the sheet. Moreover, the skew correction ability is dependent upon the rigidity of the sheet. Specifically, a thick paper with high rigidity may actually thrust through the nip and idler roller pair as the sheet is forced against the nip and idler roller pair. While this problem may be avoided, such avoidance generally takes the form of additional equipment incorporated into the machine thereby increasing the cost and complexity of the machine.
Other automatic registration systems avoid the above problems by pivoting and translating the entire nip and idler roller assembly. In some of these devices, the skew of a sheet is first detected. Then, the nip and idler roller assembly is pivoted by a de-skew motor to match the detected skew condition prior to grasping the sheet with the nip and idler roller assembly. Once the paper is grasped by the nip and idler roller assembly, the nip and idler roller assembly are pivoted by the de-skew motor into a de-skewed position. The nip and idler roller assembly and the de-skewed sheet are then translated by a lateral motion motor to provide lateral alignment of the sheet.
In other systems, the sheet may be grasped by a nip and idler roller assembly while the nip and idler roller assembly is in a home position. Accordingly, the sheet is grasped in a skewed and laterally offset position with respect to the nip and idler roller assembly. The sheet and nip and idler roller assembly are then rotated and translated for de-skewing and lateral alignment of the sheet. This results in the nip and idler roller assembly being moved to a skewed position while the sheet is properly aligned. Then, after the sheet has left the nip and idler roller assembly, the nip and idler roller assembly is returned to the home position. In these systems, the skew sensors may be located before or after the nip and idler roller assembly.
The above discussed automatic registration systems are very effective in correcting skew and lateral offset. Nonetheless, there are some drawbacks associated with the above systems. For example, the motors used to effect the process motion and the translation (i.e. the process motor and the lateral motion motor) must be pivoted along with the nip and idler roller assembly. The pivoting of the extra mass necessitates a larger motor to provide the pivoting movement in the allotted time.
The problem of pivoting the additional mass is compounded by any distance between the mass and the pivot axis. Specifically, the pivot for the registration system is generally located underneath and toward the middle of the transfer path. Thus, the pivot axis is toward the middle of the transfer path. The motors, however, are located at the side of the transfer path. This separation creates a mechanical disadvantage both when starting the rotation and when stopping the rotation. The additional momentum that thus results necessitates more power from the motor used to provide the pivoting movement.
Of course, in view of the speed of many modern machines, even a slight increase in the mass being moved may necessitate a significant increase in the power, and therefore the size of the de-skew motor, to achieve the necessary movement within a very short time span.